Answer
$\therefore$ $CH_{3}Br$ will be the unknown gas
Work Step by Step
$Rate\propto \frac{1}{\sqrt M}$
$Rate_{CH_4} = 2.4\times Rate_{unknown}$
$\frac{Rate_{CH_4}}{Rate_{unknown}} = \frac{\sqrt M_{unknown}}{\sqrt M_{CH_4}}$
$\sqrt M_{unknown} = 2.4\times 4 $
$\sqrt M_{unknown} = 9.6 $
$M_{unknown} = 92.16 g$
$\therefore$ $CH_{3}Br$ will be the unknown gas
